Microporous membranes are used in many fields. The membrane can be used in fluid treatments as, for example, a supporting base of a reverse osmosis membrane or a nanofiltration membrane, an ultrafiltration membrane, and a microfiltration membrane. In other fields, the microporous membrane is used as a battery separator. The reverse osmosis membrane and the nanofiltration membrane are capable of removing salts and low-molecular-weight organic matter, and therefore are widely used for desalination of sea water or brackish water, in the semiconductor industry, or in ultrapure water production for medical use. Further, the ultrafiltration membrane and the microfiltration membrane have a particle removal characteristic, and are used as a final filter in ultrapure water production, in beer or wine filtration, and for various water and wastewater treatments. Particularly, the use of membranes for water purification has increased, attracting attention.
Water purification using ultrafiltration membranes or microfiltration membranes have easy maintenance and operational management compared with conventionally known slow or rapid filtration methods, and can completely remove Protozoa Cryptosporidium and Giardia, which have been discussed recently because of their known ability to survive chlorine sterilization. However, the membrane filtering method requires a facility and operational cost proportional to the amount of fresh water generation, and the advantage in scale is small. Hence, membrane filtration actually requires a higher cost for fresh water generation in a large water purification plant than the other purification methods.
The reduction in the cost of fresh water generation can be achieved not only by decreasing the manufacturing cost of the membrane or the module containing the membrane, but also by decreasing the operating pressure or the membrane usage area by increasing the permeability performance of the membrane. In addition, such a membrane filtration method often adopts physical cleaning methods such as back-washing or air bubbling, or chemical cleaning methods using sodium hypochlorite, ozone, acid, alkali surfactants etc., so as to remove contaminants such as fine particles or organic matter. Accordingly, the reduction in the cost of fresh water generation may also be attained by improving the mechanical strength or chemical strength of the membrane to provide a long-life membrane module. Another effective method is decreasing the pore diameter of the surface of the primary membrane through which the raw water is supplied, insofar as the membrane's permeability performance is not impaired, so that membrane clogging is suppressed.
Cellulose acetate resin, polysulfone resin, and polyacrylonitrile resin have been used conventionally as material for ultrafiltration membranes or microfiltration membranes. Recently, a new membrane made of polyvinylidene fluoride resin, which is superior both in mechanical strength and chemical strength, has been developed. Phase separation is often used as a membrane-forming mechanism. As an example of phase separation, Non-patent Document 1 discloses a technique of forming microporous membranes by casting dope solutions obtained by dissolving polyvinylidene fluoride in various kinds of solutions on glass plates, and then dipping the glass plates in cold water. With this technique, when using a water-soluble N-methyl-2-pyrrolidone, which is a good solvent of polyvinylidene fluoride, the resulting membrane has a void structure on the contact surface with water as a result of non-solvent-induced phase separation, and a particle-packaging structure on the glass plates. However, this membrane becomes defective easily, has poor mechanical strength, and requires various controls in the membrane formation that decrease reproducibility in industrial production.    Non-patent Document 1: Journal of Membrane Science, Vol. 57, P. 1-20 (1991)
Another polyvinylidene fluoride microporous membrane production method discloses using thermally-induced phase separation that adopts a γ-butyrolactone or a propylene carbonate serving as a water-soluble latent solvent. The membrane formed by this method has a spherocrystal lamination structure (see Patent Document 1). However, in this structure, the separation depends on the voids between the spherocrystals, and therefore there is a need to precisely control the size of the spherocrystals, which is usually very difficult. Consequently, this structure tends to have a broad separation characteristic. Moreover, in such a structure formed by the bond between spherocrystals, the tensile strength at break and the tensile elongation at break tend to be lower than those having a three-dimensional, net-like structure.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-320228
Still another polyvinylidene fluoride porous membrane production method discloses the use of thermally-induced phase separation using a water-insoluble latent solvent such as diethyl phthalate or dioctyl phthalate (see Patent Document 2). This method subjects a polymer, a latent solvent, and an inorganic fine powder to melt-mixing, shapes the mixture into a membrane form, and cools the shaped membrane. The latent solvent and the inorganic fine powder are then sequentially extracted to complete a polyvinylidene fluoride porous membrane. This porous membrane does not have a void structure, and has a high tensile strength at break, a high tensile elongation at break, and superior mechanical strength. The porous membrane has a three-dimensional, net-like structure comprised of homogeneous continuous pores. However, the permeability performance of this homogeneous structure membrane significantly decreases as the structure is made denser to further improve the separation performance. Furthermore, this production method causes problematic environmental damage through the use of dioctyl phthalate, a suspected endocrine disruptor, as well as because of the need for an extra solvent or strong alkali to extract the latent solvent or inorganic fine powder. Additionally, there is considerably increased cost due to the many production steps and the solvent isolation process.    Patent Document 2: Japanese Patent No. 2899903
Still yet another vinylidene fluoride microporous membrane production method discloses that the polymer phase forms an isotropic net-like structure, which branches out three-dimensionally in arbitrary directions. In this structure, the void portions are formed between the net-like structure of the polymer phase, and are in communication with each other to form a percolation structure (see Patent Document 3). The average pore size in the surface of this net-like microporous membrane, as measured with a scanning electron microscope, is smaller than the average pore size in the internal structure, and is therefore reportedly able to prevent impurities in the liquid or gas from entering into the membrane. However, there is no teaching of the means for fabricating such a structure.    Patent Document 3: Japanese Unexamined Patent Publication No. H11-319522
Patent Document 4 discloses a technique of forming a separate active layer made of a cross-linked polyamide thin membrane on the outer layer of a polysulfone porous hollow fiber membrane having a net-like structure. However, since this method forms a polyamide thin membrane by way of interfacial polymerization on the outer layer of the polysulfone porous hollow fiber membrane prepared in advance, the membrane material etc. must not be touched until the interfacial polymerization reaction is completed. This is at least one hindrance preventing the method from being performed in an industrial manner. Moreover, since this method forms a thin membrane as a separate post-process on the surface of the previously-formed supporting membrane, there is a greater risk of separation of the thin membrane from the supporting membrane, or a risk of cavity generation.    Patent Document 4: Japanese Unexamined Patent Publication No. H07-284639